As a polyvinyl alcohol (hereinafter referred to as PVA) having reactive unsaturated double bonds introduced into its molecule, one obtained by introducing unsaturated double bonds to side chains of PVA by subjecting PVA or modified PVA to post-modification with a reactive molecule containing a polymerizable double bond (e.g. Patent Document 1), one obtained by preparing a polyvinyl ester copolymer having protected ethylenic unsaturated double bonds and then removing the protection (e.g. Patent Document 2) or one having unsaturated double bonds introduced to the molecular terminals of PVA by using an aldehyde as a chain transfer agent (e.g. Patent Document 3) has, for example, been known.
Patent Document 1: JP-A-04-283749
Patent Document 2: JP-A-2001-072720
Patent Document 3: JP-A-2004-250695